The present invention relates to a higher carboxylic acid triester of adamantane triol as a novel compound and a synthetic lubricating oil containing the same. More particularly, the present invention relates to a higher carboxylic acid triester of adamantane triol which is a novel compound useful as a constituent of high-performance synthetic lubricating oils excellent in both of the low-temperature characteristics and high-temperature characteristics as well as to a high-performance synthetic lubricating oil containing the compound which can be used quite satisfactorily as an engine oil, gear oil, hydraulic working fluid, grease, vacuum pump oil, bearing-impregnating oil, gas turbine oil and the like.
Synthetic lubricating oils known in the prior art include polyolefins such as oligomers of .alpha.-olefin, polyisobutylene and the like, organic ester compounds such as diesters, e.g., di(2-ethylhexyl) sebacate, alkyl esters of pentaerithritol and the like, polyphenyl ethers such as m-bis(m-phenoxyphenoxy) benzene and the like, phosphate esters such as tricresyl phosphate and the like, polyalkylene glycols such as polypropylene glycol and the like, silicone fluids, perfluoroalkyl ethers and so on.
These synthetic lubricating oils have unique characteristics not possessed by any mineral oil-based lubricating oils so that they are used in respective applications in accordance with the particular characteristic properties. It is a trend in the machinery of recent years that the performance of machines is being upgraded increasingly and machines are operated sometimes under extremely adverse conditions so that demand for synthetic lubricating oils is expected to grow more and more also in connection with the problems of material saving and prevention of environmental pollution. In this regard, active investigations are being carried out to develop a synthetic lubricating oil having further improved characteristics.
Turning now to the derivatives of adamantane or, in particular, ester compounds derived from adamantane, Japanese Patent Publication No. 46-22465 discloses a diester of adamantane-1,3-diol represented by the general formula ##STR1## in which R.sup.4 is a hydrogen atom or an alkyl or cycloalkyl group having 1 to 20 carbon atoms and R.sup.5 and R.sup.6 are each an alkyl or cycloalkyl group having 1 to 20 carbon atoms.
These ester compounds of adamantane, when used as a constituent of a synthetic lubricating oil, have a defect in respect of the viscosity-temperature characteristics although excellent heat resistance can be obtained therewith. For example, a diester of adamantane-1,3-diol, of which the groups denoted by R.sup.4 and R.sup.5 in the above given general formula (I) are each a methyl group and the group denoted by R.sup.6 is an octyl group, has a kinematic viscosity of 7.1 centistokes at 100.degree. C., viscosity index of 95 and pour point of -50.degree. C. or below so that this diester compound could be used as a high-performance synthetic lubricating oil if each of the viscosity at high temperatures and the viscosity index would have a still higher value.